Laser radars inherently offer higher resolution than microwave radars, and most applications for these optical wavelength radars stem from this advantage. Laser radars offer fundamental advantage over other light sources in that they have a high degree of coherence--both spatial and temporal. The spatial coherence of the output radiation of a laser is determined by the type of oscillating mode supported in the laser cavity. Where only axial modes exist, and the end walls of the optical cavity are plain circular mirrors, the resulting radiation is emitted in a circular Fraunhofer diffraction pattern (known optically as an AIRY pattern). The angle between the central maximum (bright spot) and the first null (dark ring) is normally quite small.
In the microwave region (except for deliberate noise sources) radar transmitters have excellent time coherence (at least over short times). Unlike laser sources, very few multimode effects are significant in the microwave region, since most resonators are of the order of a wavelength in size. Spatial coherence is also much closer to the diffraction limit in microwave radiators than in corresponding optical radiators, again principally from dimensional considerations.
Lasers radars can be operated with either an envelope or a coherent detection system. Envelope detection is incoherent in that it does not preserve carrier-phase information in the detected signal but it is generally simpler and puts less constraints on laser coherence than coherent or heterodyne detectors. Unlike microwave video detectors, laser envelope detectors can for a short-pulse modulation and low background noise operate as quantum-limited devices and give essentially the same detectivity as coherent detectors.
Coherent detection in a laser radar requires good spatial coherence over the receiver aperture and a stable reference signal (generally offset in frequency from the transmitted signal) with which to mix the signal received from the target. Where all restraints can be met, coherent detection not only gives Doppler information on the target but also offers a means for obtaining quantum-limited detection in the face of relatively high background noise. This method of obtaining near-ideal detectivity is applicable as far into the infrared (IR) region as one can make an optical mixer. Unfortunately because of the spatial-coherence requirement, coherent detection severely limits the maximum size of the system aperture (optical antenna) and also the field of view (FOV).
Obviously, laser radars operate at a disadvantage, compared with microwave radars, in search functions. In the past, many laser radar applications required only a minimum of search. In many cases searches could be performed by supplementary sensors, such as microwave radars or passive optical or IR detection. A classic and very effective example of an optical search supplement is the use of a simple laser range finder, where the observer merely looks for a target before firing a laser.
When working against an extended target, laser radars, being exceedingly narrow beam systems, require some form of controlled beam steering or scanning. Laser trackers, like microwave trackers, require precisely controllable beam steering over small angular regions. Lasers that are required to perform independent search functions must scan the narrow laser beam over a wide region, generally very rapidly. Both mechanical and electrical beam-steering and scanning techniques are utilized as well as hybrids of the two. Because of the relatively small diameters (one centimeter) of laser beams, for many applications one of the most effective scanners utilized is a relatively simple rotating mirror. Operating in an evacuated enclosure, mirrors with rotational speeds greater than 100,000 RPM are commercially available.
Since the advent of lasers, considerable effort has been spent on the development of electronic beam-steering techniques for optical systems. The normal microwave phrased-array system of individual phase shifters at each discrete element of a radiating array is not practical in the optical region, simply because of dimensional considerations. It is virtually impossible to construct such a phased array with .lambda./2 spacing between elements when .lambda. is about 10.sup.-4 centimeters. If the element spacing becomes appreciably wider than .lambda./2 without a corresponding increase in the size of the individual element apertures, more and more of the power is radiated in grating lobes rather than in a single main beam. If the element apertures increase in proportion to the element spacing the allowable scan angle decreases because of the sharpness of the element pattern.
Because direct electrical deflection of a light beam over wide angles is relatively difficult to obtain, the technique which has been developed combines a laser with a cathode ray tube and achieves light-beam deflection indirectly in response to electron beam deflection as described in U.S. Pat. No. 3,992,681. This device differs from other methods of light deflection in that the light is not actually scanned. Rather, laser light is generated so that it emerges from the resonator with the desired direction of propagation. This is accomplished by selectively directing the laser pump radiation through the active medium.
In order for the scan laser to work, a laser medium (solid, liquid or gas) capable of high-gain operation in a high-numerical aperture is required. With a laser that provides a single-path gain of 1.15 in an aperture greater than f/20 and with the state-of-the-art optical engineering plus a reasonable economical electron-beam system, about 10.sup.5 spots per field can be realized.
The scan laser approach allows an optical radar to transmit a signal at a varying angle. In a homodyne or heterodyne receiver the local oscillator must be coordinated with the scan laser in the transmitter such that the output of the local oscillator impinges upon a detector at the same time and the same position that the signal reflected from the target returns to the receiver. If the reflected signal from the target takes, for example, 50 milliseconds to return, it is only necessary for the local oscillator to impinge upon the detector at the expected time of return and at the expected angle of return.
Depending upon the range uncertainty of the target, it will be necessary for the local oscillator to remain focused upon the detector for a longer duration than the expected time of return. The rate of scan for the transmitter will depend upon the amount of time that the local oscillator must remain focused for a given return signal.
In view of the above problems it is necessary to have a system that has the capability to rapidly acquire, recognize, track and perform simultaneous guidance functions for a multiplicity of weapons against a multiplicity of targets. It is necessary to develop a receiver that will allow the transmitter to scan a greater number of beam positions per second while maintaining a sufficient range uncertainty in the receiver.